Although it can be used for other devices, a USB Y-cable is most commonly used to power an external hard drive with a computer or laptop. This type of cable provides more power to an external device than a traditional USB connection.
A USB Y-cable splits off into two USB ports on one end, which allows it to plug into two USB slots. Plugging the cable into two USB slots instead of one is what draws additional power from the computer or laptop. While this connection is necessary for some external hard drives to operate, others simply benefit from faster data transfer speeds.